1. Field of the Invention
The present invention relates to ventilation fans, and in particular, high velocity exhaust fans for exhausting atmosphere containing airborne contaminants to be diluted by ambient air.
2. Description of the Prior Art
An "upblast" fan is described in U.S. Pat. No. 4,806,076, Andrews, issued Feb. 21, 1989. The exhaust fan described in this patent has a lower portion and an upper portion with a radial fan in the lower portion. The upper portion of the housing includes two somewhat parallel passageways defining first and second flow paths. The walls forming these passageways are shaped as sectors of conical sections. A wind band is provided at the top end of the two passages at the outlets thereof to provide an entrainment of fresh air to mix with the gases exhausting from the two passageways. Access to the fan motor is centrally through the gap formed in the upper portion between the walls forming the two passageways. Although ventilation fans in accordance with U.S. Pat. No. 4,806,076 are acknowledged to be an improvement over the prior art fans, there are constraints in the design of the fan which limit the performance and efficiency thereof. For instance, the diameter of the radial fan is limited as well as the motor size. The diffuser efficiency is also limited due to the limited space at the periphery of the radial fan.